The present invention relates to a transmission ratio control system in which engine power is established by a manually controllable fuel supply means and a hydrostatic transmission ratio is varied by comparing engine speed with engine torque as derived from sensing system pressure in the hydrostatic transmission to provide a predetermined engine speed in which minimum fuel is consumed.
An infinitely variable ratio power transmission device, such as a hydrostatic or hydromechanical transmission, is capable of adapting variable input speeds to variable output speeds in any combination over the operating range of the transmission and engine. Once such transmissions are installed in conjunction with a variable speed engine in a mobile vehicle, such as a truck or tractor, it is often desirable to operate the engine at various regulated speeds which are dependent upon power demand in order to obtain certain operating characteristics, such as maximum fuel economy, maximum power or maximum engine life. Engine speed can be controlled by a ratio control valve responsive to engine speed and engine torque in a manner to vary the transmission ratio to control the engine speed for minimum fuel consumption for a predetermined power requirement.
Generally, infinitely variable hydrostatic and hydromechanical transmissions require a mechanical input signal to set the desired input or engine speed. This signal is generally coordinated with a signal to the engine fuel control so that the engine will be operated at the most economical power/speed combination. The correct relationship is difficult to achieve and is subject to misadjustment with mechanical linkage. Mechanical linkage must be designed for each style of vehicle.
In the prior patent, Nyman, U.S. Pat. No. 3,733,931 assigned to the assignee of this application, a hydrostatic transmission ratio is controlled in a hydromechanical drive by a ratio control valve which is responsive to an engine speed signal and a mechanically applied horsepower signal. In U.S. Pat. No. 3,583,154, a hydrostatic transmission ratio is controlled by a ratio control valve which is responsive to an engine speed signal and responsive to a mechanically applied torque signal and, at a higher range of throttle settings, the ratio control valve is responsive to compressor discharge pressure which is proportional to the speed thereof.
In the prior application of Pollman et al., Ser. No. 352,660, filed Apr. 19, 1973, now U.S. Pat. No. 3,855,793, granted Dec. 24, 1974, and assigned to the assignee of this application, a hydrostatic transmission ratio is controlled by a ratio control valve which is responsive to an engine speed signal and a fluid signal representing horsepower as determined by a fuel rail signal.
The instant application includes a proportional linkage structure wherein system pressure is applied to a movable member at a variable location, depending upon output speed of the hydrostatic transmission. Proportional linkages used in control of transmissions are shown in the U.S. Pat. Nos. to Kacer 1,981,805, Thoma 2,976,685 and Molly 3,647,322. The proportional linkages in these patents do not include the provision of modifying the application of a force resulting from system pressure of a transmission which represents output torque with output speed and, specifically, by modification from the position of the swashplate of a variable displacement element of the transmission.